1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and provides a technique of uniformly depositing an yttrium-stabilized hafnium oxide used as a highly dielectric insulating film on the surface of a sample having a structure with a high aspect ratio and a technique relating to a deposition method capable of highly accurately controlling the addition amount of yttrium. The technique concerns formation of a capacitive insulating film of a DRAM (Dynamic Random Access Memory) used as a memory for accumulating charges to record information in a capacitor and a gate insulating film of an MIS transistor.
2. Description of the Related Art
A DRAM memory cell area per bit has been reduced for higher integration density. On the other hand, capacitors, which require a predetermined capacity to avoid a read error, have been developed to have a three-dimensional capacitor structure and to reduce the thickness of a capacitor insulating film for the reduced bit areas. While various structures have been proposed for the three-dimensional capacitor structure, a trench shape is predominant at present. However, in the current processing technology, the aspect ratio is limited to about 20 to 30. The thickness of the capacitor insulating film has been reduced and the capacitance per unit area has been increased as the generations have passed. However, since silicon dioxide for the insulating film used so far has a specific dielectric constant as low as 3.9, direct tunnel leak current becomes conspicuous along with reduction of the film thickness, making it difficult to satisfy a refresh request. Accordingly, it has become essential at present to apply a highly dielectric insulating film capable of suppressing the direct tunnel leak current.
At first, a noteworthy material is tantalum pentoxide having a dielectric constant of about 50 in a crystalline state. In an MIS (Metal Insulator-Semiconductor) structure, tantalum pentoxide has been effective since a good boundary with a lower electrode polysilicon can be obtained. However, there may be a high possibility that no stability can be obtained at the boundary between tantalum pentoxide and a lower electrode titanium nitride in an MIM (Metal-Insulator-Metal) structure which is advantageous for the reduction of thickness because depletion in the lower electrode can be suppressed. This is because titanium dioxide is lower in energy than tantalum pentoxide and stable. Accordingly, tantalum pentoxide is reduced to form titanium dioxide in a case of forming a film using tantalum pentoxide on a lower titanium nitride electrode. In view of the above, an application of a different highly dielectric insulating film has been necessary.
Hafnia has a dielectric constant of about 24 which is higher than silicon dioxide. Also, hafnia gives relatively satisfactory boundary with the lower titanium nitride electrode and has high affinity with a silicon process. Furthermore, the film deposition method for hafnia has been established. Thus, hafnia has been extensively studied. With a view point of reducing the equipment investment cost or research and development cost for the material, it is desirable that the type of the material applied is not changed for a long period of time. The increase of the dielectric constant due to an addition of an element is a key to extend the applicable period of time of hafnia. At present, yttrium has been noted as an additive element for increasing the dielectric constant of hafnia. Yttrium has a smaller ionic radius compared with elements such as hafnium or O, the dielectric polarization increases due to the change of the coordination number when yttrium is added to hafnia and, as a result, the dielectric constant increases. It has also been confirmed that yttrium-stabilized hafnia can be used to reduce the film thickness compared with hafnia in capacitor evaluation. Also, products using the yttrium-stabilized hafnia are expected to be put into market. At present, a DRAM capacitor has a trench shape with an aspect ratio as high as 20 or more, and films should be deposited conformally also to such a structure. An atomic layer deposition method as a film deposition method for hafnia or the like allows for conformal film deposition and has excellent thickness controllability. However, the film deposition technique for yttrium-stabilized hafnia by the method described above has not yet been developed.
International Electron Device Meeting (IEDM) 2003, Proceeding “A Model for Al2O3 ALD Conformity and Deposition Rate from Oxygen Precursor Reactivity”
In a case of depositing a binary material such as an oxide of a specific element, for example, alumina by an atomic layer deposition method, a precursor of specific element compound and an oxidant are supplied alternately to form one atomic layer one by one on the surface. At first, a specific element precursor is supplied on a sample surface where OH groups are exposed to replace the OH groups. In this case, atomic layer deposition is attained when all the OH groups on the sample surface are replaced. For this purpose, at least the time of exposing the sample to the specific element precursor may be made sufficiently longer. Then, the oxidant is introduced to oxidize the sample surface replaced with the specific element again. In this case, it may suffice that all the groups bonded with the specific element are replaced with the OH groups. This can be attained at least by taking a sufficient time to replace the sample with the oxidant. As described above, in the existent atomic layer deposition method, since deposition is conducted by repeating the steps of completely replacing the surface of the sample with the introduced material, it may suffice to at least ensure the material introduction time sufficiently. However, in the film deposition of yttrium-stabilized hafnia, it is not sufficient to merely increase the supply of the precursor. Even when the yttrium compound precursor is introduced after introducing the hafnium compound precursor and completely replacing the OH groups, adsorption sites are not left. Thus, the replacement reaction does not proceed. Further, in a case where yttrium may be merely added to hafnia and the accuracy and the uniformity of the addition amount do not cause a problem, it may suffice to separately deposit and laminate yttrium oxide. However, since the dielectric constant of hafnia formed by adding yttrium intended for the use of the insulating film for the DRAM capacitor is changed depending on the addition amount of yttrium, it is necessary to control the addition amount of yttrium at a high accuracy in order to obtain a high dielectric constant over the entire film.